Preordering in general has a lot of baggage attached, due to people having experiences with games where they preordered and then the game was a buggy mess on release, or just failed hard to live up to expectations. But that's more a problem with releases and setting expectations than it is with preordering. The way I see it, preordering is essentially allowing the customer to say, "I trust you enough to deliver a good product that I will take this product the second it is available." For those who would rather jump into a product and try it for themselves, rather than stand back and sift through reviews, preordering is just an extension of that approach to trying a product; it allows the customer with that kind of mindset to bypass the hassle of ordering based on the company's schedule, rather than their own; with preordering, they can order when they have the time to do it and then the game will be available to them without hassle the moment it's released.
And this mindset, though it might seem odd to those of us who are commonly skeptical and cautious about what we buy, it makes more sense in the context of a game like this, years in, with lots of DLC (as opposed to a brand new title); by this point, if you've been following the game since its release (or close to it) and you don't already trust Maxis to deliver on the expectations they set, you're probably never going to for TS4. For those for whom that trust is established, it makes perfect sense for them to want to preorder when they can, for content that appeals to their wants, since they already have a high level of confidence that they are going to be satisfied.